Security laminates are traditionally used to protect documents or packages to ensure that the underlying items are not altered by containing an authentification feature making them difficult to counterfeit. Security laminates are particularly useful on identification cards such as driver's licenses, ID-cards and passports, and on other important documents such as certificates of title. Security laminates are also useful as tamper proof seals on medications, video cassettes, and compact discs. Five features are particularly important when producing and using security laminates. First, once applied to an article it is important that the laminate is difficult to remove to ensure that the underlying item is not altered or subjected to tampering. Second, a desirable laminate is difficult if not impossible to duplicate by counterfeiters. Third, if tampering occurs it is important to quickly and accurately recognize an altered or counterfeit laminate. Fourth, it is important that manufacturing costs of the laminates are not prohibitively expensive. Fifth, when used on articles such as identification cards, it is important that the laminate has sufficient durability to withstand harsh treatment.
Holograms have, due to the difficulty in making and reproducing them, become a common authentication feature on items like credit cards, driver's licenses and access cards. Holograms have also been used as security features on high end products, making it harder to counterfeit these products.
A major advance in display holography occurred in 1968 when A. Benton invented white-light transmission holography. This type of hologram can be viewed in ordinary white light creating a “rainbow” image from the seven colours which make up white light. Benton's invention made mass production of holograms possible using an embossing technique. With this technique holographic information is transferred from light sensitive glass plates to nickel embossing shims. The holographic images are “printed” by stamping the interference pattern onto plastic. The resulting hologram can be duplicated millions of times inexpensively, although master-shim rolls will need to be regularly replaced. Therefore, embossed holograms are now being used by the publishing, advertising, banking and security industries. The difficulty in making and reproducing holograms has made them a common authentication feature on security items such as credit cards, driver's licenses and access (identification) cards. Transparent holograms are often used for security applications such as identification or access cards, where it is desired that information positioned behind the hologram remains visible to the unaided eye.
The most common method of creating a hologram is to create a grating pattern in a surface so that particular structures become visible upon diffraction of light in the grating. U.S. Pat. No. 3,578,845 describes how diffraction gratings are typically generated. Typically, the diffraction patterns are embossed into a thermo-formable substrate such as an embossable polymer film. This process is performed by pressing a heated stamp made from a hard material to engrave the desired grating from the contact surface of the stamp onto the embossable substrate.
Diffraction requires that the medium the grating is made of and the media bordering the grating have a difference in refractive index index. The larger this difference, the brighter the diffraction will appear. To create the highest diffraction in transparent holograms, the grating is coated with a thin film of transparent material having a high refractive index (HRI).
While the grating can be embossed into the substrate material with a stamp, a more common, economical method, is the use of continuous embossing systems. Such embossing systems are described for example in U.S. Pat. Nos. 4,913,858 and 5,164,227. In these methods the grating structure is engraved into the surface of a roll, which continuously presses its surface pattern into the web type substrate passing between the embossing roll and a backside roll. In order to obtain the grating in the substrate's surface, the thermo-formable layer on this surface is heated. This can be achieved either by preheating the substrate to the required temperature, or by heating the embossing roll.
WO 2007/027619A2 discloses a translucent holographic film comprising: an adhesive layer; an embossed layer, the embossed layer including an embossed side defining ridges having peaks pointing toward the adhesive layer.
Transparent holograms are commonly produced by using vacuum evaporation techniques to coat an embossed polymer with a transparent HRI-coating. Moreover, to prevent tampering the resulting hologram is often embedded by lamination within multiplex identity documents. This requires that not only the support/base of the hologram adhere to a contiguous film or layer within the multiplex system making up the identity document, but also that outermost surface of the HRI-layer also adhere to a contiguous film or layer within the multiplex system making up the identity document.
Moreover, adhesion of the outermost surface of the HRI-layer to a contiguous film, layer or card should occur without substantial loss of the brilliance of the hologram to prevent loss of contrast.
Commercially available transparent embossed holograms coated with an inorganic HRI-material do not lend themselves to tamper-proof interlamination in the multiplex system making up a multiplex identity document.